Recently, hybrid electrically-driven vehicles using a motor and an engine mounted thereon or electric cars driven by a motor alone are increasing in the effort of reducing CO2 emission. These electrically-driven vehicles using a motor mounted thereon include an inverter to drive the motor and a battery serving as a power supply in addition to the motor.
In these electrically-driven vehicles, the battery is charged by regenerative power generation with the aim of extending a cruising distance or with the aim of suppressing an increase of fuel consumption by the engine for power generation. In regenerative power generation, energy that is otherwise consumed as heat generated at a brake is extracted as electric energy. Hence, the cost incurred or fuel consumed by this power generation is zero. It is therefore desirable to store the power generated by regenerative power generation in the battery as much as possible.
On the other hand, many of the batteries mounted on the electrically-driven vehicle have a property that the life becomes shorter when charged with an overcurrent or overcharged. Hence, when the batteries are charged, the processing to protect the batteries from overcharging or the like is necessary.
In order to overcome this problem, Japanese Patent No. 3751736 (PTL 1) discloses a technique, according to which SOC (State of Charge) detection means for detecting an SOC (hereinafter, referred to also as a storage amount) of the battery is provided. In a braking mode in which regenerative power generation is performed, regenerative power generation is stopped when the SOC of the battery is close to a full charge and the mode is switched to countercurrent braking. In contrast to the regenerative power generation by which the battery is charged with power generated by the motor, battery power is consumed by the countercurrent braking because the motor is driven by power running. Hence, there is no risk of overcharging the battery. In this manner, the technique disclosed in PTL 1 prevents overcharging of the battery by regenerative power generation.
Also, JP-A-2003-164002 (PTL 2) discloses a technique, according to which SOC detection means for detecting an SOC of the battery is provided. In a case where it is determined that the battery cannot be charged (for example, when an SOC of the battery is close to a full charge), a three-phase short circuit is applied by short-circuiting input terminals of the motor. By applying the three-phase short circuit, power generated by the motor is consumed within the motor and is not charged to the battery. Hence, there is no risk of overcharging the battery. By configuring in this manner, overcharging of the battery by regenerative power generation is prevented.
Further, JP-A-9-47055 (PTL 3) discloses a technique, according to which overcharging is prevented by applying a three-phase short circuit when a synchronous generator is under weak field control, that is, when an inductive voltage generated by the motor is large in comparison with a voltage across the battery. In a case where an inductive voltage generated by the motor is large in comparison with a voltage across the battery as above, an amount of current to be generated cannot be controlled by the inverter. Hence, overcharging is prevented by applying a three-phase short circuit.